This invention relates to a paper feed control apparatus and method for a dot printer through which a paper feed operation is selectively set in either a one-pitch feed mode or a multi-pitch feed mode to feed, over one pitch or a preset number of pitches, a sheet of printing paper in a direction perpendicular to a printing direction.
In general, the dot printer is adapted such that, while moving a printing head-mounted carriage at an even speed along a platen with a sheet of printing paper set thereon, it can read, from a character generator, a dot pattern corresponding to a character code input externally, and then print the character in question on the sheet of printing paper. As shown in FIG. 1, for example, the dot printer has eight printing needles 2 in a longitudinal array. Accordingly, therefore, the longitudinal array of a dot pattern character printed by the printing head 1 is comprised of eight dots.
A simple character, such as a numeral, for example, even though comprised of only eight dots in a longitudinal array, can be sufficiently legible. However, a complex character such as a Chinese character, is, if so comprised, not sufficiently reproduced to express the features of the character. Therefore, after printing has been completed once by the printing head with eight needles, the character corresponding to the same input character code is re-printed on substantially the same area of the sheet of printing paper, with the sheet fed at one pitch equal to one half the dot-to-dot distance. In this way, if substantially the same character is printed on the two areas with the sheet fed or displaced at one pitch, the complex character, such as the Chinese character, can be exactly expressed as if the dots in the longitudinal array were comprised of substantially 16 dots and, furthermore, the simple character, such as the numeral, can be represented beautifully and precisely. When a line feed is to be effected after the per line printing of the characters has been completed, the sheet of printing paper is fed, over the number of pitches, at a time corresponding to a distance of a sum of one character height and the height of a line-to-line spacing. For example, with the line-to-line spacing set to eight pitches corresponding to one half of the character height, the sheet is fed over 24 pitches at a time as a paper feed. Here, "a one-pitch feed mode" refers to a mode in which the sheet of printing paper is fed at one pitch at a time, and "a multi-pitch feed mode" refers to a mode in which the sheet of printing paper is fed over a predetermined number of pitches at a time.
In order to feed the sheet of printing paper, a stepping motor, used as a paper feed motor, is coupled to a platen through a power transmission mechanism, such as a gear, and rotated in response to a step pulse for the purpose of driving the platen. The gear ratio of the coupling gear is so adjusted that, when the stepping motor is rotated by one step upon receipt of a one step pulse, the sheet of printing paper is fed over a distance corresponding to one pitch. Thus, when the stepping motor receives a one step pulse, the sheet is fed by one step at a time, while, when the stepping motor receives 24 step pulses, the sheet is fed over a distance corresponding to the 24 step pulses: 24 steps at a time for line feed.
Since the one-pitch and multi-pitch feed mode are selected as required, the paper feed control device for a dot printer poses the following problem. That is, in order to increase the entire printing speed, the rotation speed of the stepping motor is increased in the multi-pitch feed mode during the paper feed operation to shorten the time required for paper feed. As appreciated from the above, the multi-pitch feed mode is different from the one-pitch feed mode with respect to the speed at which the sheet is fed, i.e., at one pitch at a time. In other words, they are different from each other with respect to the rotation speed of the stepping motor. As a result, there is a risk that the distance over which the sheet is fed in the respective feed mode will differ from pitch to pitch. More specifically, where the stepping motor is rotated at a high speed, a greater load is applied to the motor and power transmission mechanism, with the result that there is a tendency for the pitch to be somewhat lengthened. A relative gradual displacement occurs between when the sheet is fed, at first, in the one-pitch feed mode, in the multi-feed mode over a distance corresponding to 16 pitches as indicated by broken lines in FIG. 2A and then in the one-pitch feed mode, and when the sheet is sequentially fed in the one-pitch feed mode as indicated by solid lines in FIG. 2B. Thus, an error or discrepancy DO between a diagonal dot array of eight dots (as indicated by open circles) printed after the paper feed has been effected in the multi-pitch feed mode, as shown in FIG. 3, and a diagonal dot array of eight dots (as indicated by solid circles) printed after the paper feed is performed, one pitch at a time, in the one-pitch feed mode. This may result in both an illegible dot pattern character and in a lowered printing quality.